It's one of the tropes of our times, that the inheritance tax is such a spiffingly new and inventive idea that it must be progressive. In one way of course it is new, starting in 1986 (with the origin as Estate Duty in 1894), in another, it's really rather old.
Feudal England worked on the basis that everything, most especially the land, belonged to the King. Whilst you might have an estate, even the hereditary right to one, you only ever held a lifetime interest in it. At your death, your inheritors would pay a fee to the next lord up in the fedual chain. This progressed all the way to the top, where those who held lands directly from the Monarch paid him at their own moment of inheritance. There were other such details, such as heriot, the right of a lord to seize the best animal or clothes of a serf from his estate.
Fortunately we got rid of such things as the medieval system faded away: land holdings became freeholds, owned absolutely, property rights actually meant that things were inheritable rather than held only for life.
Until, of course, the system of inheritance tax came along again. What is ours is not ours to do with as we wish again, to dispose of as we please. Our inheritors must pay again a fee, to the State this time instead of the feudal lord, for the enjoyment of our property, what we have built and paid for.
Thus, far from being progressive, the inheritance tax is in fact medieval, positively feudal: we're now simply regarded as serfs of the State, ripe for the plucking upon our passing.